[libc++] Adds back_insert_iterator::__get_container.

Adds a `__get_container` member as suggested by @Quuxplusone in D110497.

Includes  s/_LIBCPP_INLINE_VISIBILITY/_LIBCPP_HIDE_FROM_ABI/.

Reviewed By: Quuxplusone, #libc, ldionne

Differential Revision: https://reviews.llvm.org/D110573